1. Field of the Invention
The present invention relates to an injection molding machine including a position detector of a die opening/closing device.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 5-437 discloses a technique in which a position detector for detecting a position of a die opening/closing device is provided on a machine frame of an injection molding machine and the die opening/closing device is automatically moved to a position corresponding to the thickness of a die mounted on the injection molding machine. Therefore, a conventional technique will be described with reference to FIG. 3.
A die clamping device is provided on a base 38, a base portion of a front platen (not illustrated) is fixed to the base 38, and a base portion of a rear platen 33 is placed in a rail 39 on the base 38. Here, the die clamping device is movable in the front-to-back direction so as to adjust the thickness of the die. A feeding screw 30 is formed in each of the rear ends of tie-bars 34, and the feeding screws 30 extend backward from the rear platen 33. A tie-bar nut 31 provided with a sprocket attachment is axially supported to the rear surface of the rear platen 33 so as to rotate at the constant position in the axial direction in association with a position where the rear portion of the tie-bar 34 protrudes from the rear surface of the rear platen 33. Then, the tie-bar nut 31 is threaded into the feeding screw 30 formed in the tie-bar 34.
An endless chain is wound around four tie-bar nuts 31 which are threaded into the feeding screws 30 of four tie-bars 34, and the chain is driven by a die thickness adjustment servo-motor 32, so that these four tie-bar nuts 31 rotate in a synchronization state. Accordingly, when the die thickness adjustment servo-motor 32 is driven, the tie-bar nut 31 rotates, and the rear platen 33 moves forward or backward by the pitch of the feeding screw 30. Meanwhile, when the die thickness adjustment servo-motor 32 is stopped, the tie-bar nut 31 does not rotate, and the rear platen 33 is fixed to the current position.
A measurement portion 37a of a linear scale 37 is horizontally fixed to the side surface of the rear platen 33, one end of a bar-shaped scale portion 37b is fixed to one point (a point P) on the base 38, the other end of the scale portion 37b is fitted into the measurement portion 37a, and hence the distance between the point P and one point (for example, an engagement point Q between the rear platen 33 and the tie-bar 34) of the measurement portion 37a is measured. The measurement value obtained from the measurement portion 37a is input to a numerical controller 40 of the injection molding machine. The numerical controller 40 performs a control involving with the injection molding operation, and the memory also stores an operation program in a die thickness adjustment mode. When the die clamping servo-motor 36 is driven, the cross head member 35 of the toggle mechanism moves forward or backward.
Furthermore, there is a case in which a geared motor is used instead of the die thickness adjustment servo-motor. Here, one gear engages with four tie-bar nuts, and the gear is driven by the die thickness adjustment servo-motor or the geared motor.
In the injection molding machine, the position of the die opening/closing device is adjusted in accordance with the thickness of the installed die. The position may be adjusted by a method of moving the die opening/closing device while an operator checks the movement with his/her eyes. However, since the die opening/closing device generally moves at a low speed, it takes a time for the movement, and hence the operator feels burdensome.
In order to solve this problem, according to the conventional technique, a die contact position is searched in a manner such that an injection molding machine automatically moves a die opening/closing device to a position corresponding to the maximum thickness of the die mounted on the injection molding machine and then gradually moves the die opening/closing device in a direction in which the die thickness decreases. However, since the die opening/closing device moves at a low speed, it takes a time for the movement. As a result, there is a disadvantage that a die replacement time increases compared to the case where the position is manually adjusted by the operator.
As a means for solving this problem, Japanese Patent Application Laid-Open No. 5-437 discloses a technique of detecting the position of the die opening/closing device on the machine frame by means of a position detector provided on the machine frame and automatically moving the die opening/closing device to a position corresponding to the thickness of the die mounted on the injection molding machine. According to this technique, the die opening/closing device does not need to be moved to a position corresponding to the maximum thickness of the die mounted on the injection molding machine and the die replacement time maybe shortened compared to the conventional technique.
However, there is a concern that the position detector may be contaminated by the oil scattering from the mechanical section of the injection molding machine such as the die opening/closing mechanism driving ball screw or the push mechanism driving ball screw of the die opening/closing device at the position on the machine frame provided with the die opening/closing device or the position in the vicinity thereof and hence the detection precision may be adversely affected.